The present invention relates to a control rod for controlling the power of a boiling water reactor and a method for manufacturing the same.
The control rod typically has a structure wherein a handle is attached to an axially upper part of a tie rod having a substantially cruciform cross section; a lower part support member (or velocity limiter) is attached to at an axially lower part of the tie rod; and four sheaths, each of which incorporates a reaction rate controlling material, are fixed at a lower end of the handle, an upper end of the lower part support member and ends of the substantially cruciform of the tie rod by welding. In this case, a perfect weld penetration by the TIG (tungsten inert gas) welding has been performed for welding the sheaths to the ends of the handle, the lower part support member and the tie rod.
The control rod moves upward and downward in the narrow gap secured among the fuel assemblies during operation of the reactor. Therefore, a high degree of machining precision is required in manufacturing the control rod.
However, the conventional TIG welding has such drawbacks that it requires a large amount of heat input and tends to increase deformation due to welding. Thus, in order to suppress the deformation caused by welding, a method employing a laser welding, which requires a less amount of heat input, has been proposed in Japanese Patent Laid-open No. 2000-329885.
In the aforementioned prior art literature, the perfect weld penetration is carried out in the following manner. Steps are provided on each of tips of arms of the tie rod to fit a U-shaped tip of each of the sheaths thereonto, and each of tips of the sheath are directly irradiated with a laser beam in such a manner that the axial center position of the beam is shifted from an end face of the step of the tie rod to a side opposite to the axis center of the tie rod by 0.1 to 2.0 mm.
The above-described prior art has the following problems. Specifically, since a width of the step at the tip of each of the arms of the tie rod is typically about 0.5 mm, an overlap of the step of the tie rod with the tip of the sheath is about 0.5 mm. Therefore, if an error occurs in the axial center position of the laser beam and the laser beam is deviated from the very narrow overlap portion, the sheath is heated to melt down due to a slow heat transfer of the laser beam to the tie rod, to thereby cause a welding failure.
Further, even if the irradiation position of the laser beam is located within the overlap, a contact area of the step of the tie rod with the tip of the sheath must be sufficiently large to satisfactorily transfer the heat of the laser beam to the tie rod. Therefore, the step of the tie rod must be machined to achieve a precise rectangular shape. More specifically, if an R shape (round portion) is formed at a corner of the step of the tie rod, the contact area of the tie rod with the sheath becomes small to fail to provide the satisfactory heat transfer, and such imprecision may cause the melt-down of the sheath, resulting in the welding failure.
In order to prevent the above problems, the prior art requires a high precision control of the laser beam for the prevention of the deviation of the laser beam irradiating position from the overlap and a high precision machining of the step of the tie rod. Thus, it has been difficult to simplify the manufacturing process of the control rod, and, also, the production cost has been undesirably increased in the prior art.
An object of the present invention is to provide a control rod for boiling water reactor and a method for manufacturing the control rod for boiling water reactor, whereby the manufacturing process can be simplified and the production cost can be decreased.
(1) According to an aspect of the present invention, a tie rod having a cruciform cross section is provided with steps for fixing sheaths at tips of cruciform arms of the tie rod; the tips of each of sheaths are fitted onto the steps of the tie rod, each of the sheaths having a U-shaped cross section; and each of the sheaths is fixed to the tie rod by performing a laser welding using a YAG (yttrium aluminum garnet) laser beam or a CO2 laser beam with the sheath being fitted onto the tie rod to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof. An axial center position of the beam is shifted from an end face position of the step of the tie rod at least toward an axis center of the tie rod.
In this case, the laser beam is not irradiated directly on the sheath, but firstly on a surface of the tie rod so that heat is transferred from the surface of the tie rod to the sheath which is being brought into contact with the tie rod step via the tie rod step. Accordingly, even if a small error in the beam axial center position occurs to cause a slight deviation from the target position, the heat is transferred to the sheath after passing the contact portion of the step with the sheath without fail, thereby eliminating possibility of a welding failure which is caused by the melt-down of the sheath. Thus, the present invention prevents the melt-down of the sheath to secure a good weldability without high precision control of the beam axial center position which has been performed in the conventional method. Therefore, the present invention facilitates the laser welding control as well as the manufacture of the control rod, and achieves a reduction in manufacturing cost.
(2) According to another aspect of the present invention, steps of a tie rod are formed by a drawing process, each of sheaths is fixed to the tie rod by performing a laser welding using a laser beam with the sheath being fitted onto the tie rod to achieve a continuous weld of at least a part of the tie rod in a longitudinal direction thereof.
As described in the above item (1), by shifting the laser beam axial center position toward the tie rod axis center, the melt-down of the sheath can be prevented to thereby secure the good weldability even if the tie rod step is not machined into a precise rectangular shape and thereby an R-shape (curve) or the like remains on a corner of the step. Thus, it is possible to omit a machining step from a typical control rod manufacturing method comprising a process step of forming the drawn tie rod having a substantially cruciform cross section by a drawing process and a process step of machining of the steps to achieve the rectangular shapes, thereby making it possible to manufacture a multiple of the tie rods each provided with the steps at one time by the drawing process only. Therefore, the present invention facilitates the manufacture of the control rod by a process corresponding to the machining process omitted, to thereby achieve a reduction in manufacturing cost.
(3) According to further aspect of the present invention, a step for fixing sheaths is formed on a lower end of a handle attached to an axially upper part of a tie rod; an upper edge of each of the sheaths is fitted onto the step of the handle; and each of the sheaths is fixed to the handle by performing a laser welding using a laser beam with the sheath being fitted onto the handle to achieve a continuous weld of at least a part extending along the upper edge of the sheath. An axial center position of the beam is shifted from an end face position of the step of the handle to a side opposite to the sheath.
In this case, the laser beam is not irradiated directly on the sheath, but firstly on a surface of the handle so that heat is transferred from the surface of the handle to the sheath which is being brought into contact with the handle step via the handle. Accordingly, even if a small error in the beam axial center position occurs to cause a slight deviation from the target position, there is no possibility of a welding failure which is caused by the melt-down of the sheath. Therefore, the same effect as that in Item (1) is obtained.
(4) According to a still further aspect of the invention, a step for fixing sheaths is formed on an upper end of a lower part support member or a velocity limiter attached to an axially lower part of a tie rod; a lower edge of each of the sheaths is fitted onto the step of the lower part support member or the velocity limiter; and each of the sheaths is fixed to the lower part support member or the velocity limiter by performing a laser welding using a laser beam with the sheath being fitted onto the lower part support member or the velocity limiter to achieve a continuous weld of at least a part extending along the lower edge of the sheath. An axial center position of the beam is shifted from an end face position of the step of the lower part support member or the velocity limiter to a side opposite to the sheath.
In this case, the laser beam is not irradiated directly on the sheath, but firstly on a surface of the lower part support member so that heat is transferred from the surface of the lower part support member to the sheath which is being brought into contact with the step of the lower part support member via the lower part support member. Accordingly, even if a small error in the beam axial center position occurs to cause a slight deviation from the target position, there is no possibility of a welding failure which is caused by the melt-down of the sheath. Therefore, the same effect as that in Item (1) is obtained.